1. Field
The present disclosure relates to a charging device and an image forming apparatus, and more particularly, to a roll-shaped charging device and an image forming apparatus using the same.
2. Description of the Related Art
In a conventional printer using toner, a charging member having a roll shape, a brush shape, or a blade shape to which a charging voltage is applied may be brought into contact with a photosensitive body while applying a predetermined weight to the photosensitive body. Discharging may occur in a fine gap adjacent to a contact portion between the charging member and the photosensitive body to enable charging of the photosensitive body.
For example, (Japanese Patent Publication No. 04-268584 discloses a technique in which a charging member includes an internal conductive roller and a tube roller having a portion surrounding the conductive roller and rotatably installed and the tube roller is pressed against a photosensitive body by the internal conductive roller to induce charging. Also, Japanese Patent Publication No. 2002-55510 discloses a configuration for increasing the hardness of a member corresponding to a tube.
In addition, Japanese Patent Publication No. 11-352748 describes a configuration in which a plurality of limiting rollers are installed in a conductive charging film to support the conductive charging film and limit a nip location with respect to a photosensitive body.
Furthermore, Japanese Patent Publication No. 2010-2581 describes a configuration in which a shaft-less hollow roller is positionally limited by an external casing and brought into contact with a photosensitive body.
Moreover, Japanese Patent Publication No. 8-146709 discloses a configuration in which an end portion of a hollow tube roller is fixed to a flange not in direct contact with the tube roller using a rubber ring installed around the flange. In this configuration, the flange may be connected to a through shaft, and electricity may be supplied from the through shaft via the flange.
However, in the technique in which a charging member is pressed against a photosensitive body while applying a weight to the photosensitive body and rotated to enable uniform charging, since pressure is applied to the entire charging member, the applied weight is increased, thereby facilitating abrasion of a surface of the charging member. In particular, when a solid charging roller is used instead of a hollow charging roller, pressure applied to the vicinity of a center of the solid charging roller in an axial direction may be reduced and thus it may be necessary to increase a weight applied to the charging roller, and thus a surface of the solid charging roller may be easily worn away and damaged. Also, since charging depends on occurrence of discharge in a fine gap, the charging member may be degraded due to discharge stress. In addition, when a relatively heavy weight is applied, extraneous matter, such as a carrier, may be caught in a contact portion, thereby causing scratches on a surface of a photosensitive body. Furthermore, surface filming due to a discharge generator or an external additive may lead to deterioration of image quality. Also, if a relatively heavy weight is applied, a compressed portion of a charging roller may be deformed during transfer or storage, that is, only a portion of the charging roller may be distorted. Thus, uniform charging of a photosensitive body may not be possible, thereby degrading image quality.
Furthermore, to enable uniform charging, a charging roller needs to drive or be rotated with respect to a photosensitive body at a uniform speed. However, when a relatively heavy weight is applied, smooth rotation of a charging roller may become difficult, and the charging roller repeatedly performs a stepwise operation of rotating and stopping within a relatively small amount of time. For this reason, charging of a photosensitive body in a desired state may be difficult, thus resulting in deterioration of image quality. In the technique of Japanese Patent Publication No. 04-268584, if an internal roller (or a conductive wire) is rotated, an external tube member (or a tube-shaped resistance member) may be neither uniformly driven nor driven at a uniform speed due to internal fiction. Also, in Japanese Patent Publication No. 2002-55510, which proposes the technique of increasing the hardness of a member corresponding to a tube, the hardness of a fine region corresponding to a contact portion may be increased and thus occurrence of scratches on a photosensitive body may not be suppressed. In addition, by increasing the hardness of a member, a width of a contact portion between the member and a photosensitive body may be reduced, and a charged width of the photosensitive body may be reduced, thus preventing stable charging.
In the technique disclosed in Japanese Patent Publication No. 11-352748, since a film limiting roll is included in a tube-type charging film, uniform rotation may not be possible, as in Japanese Patent Publication No. 04-268584. Also, since pressure applied to the charging film at a pressure contact (or nip location) depends on the hardness of the charging film, the applied pressure may not be stably applied.
In the technique disclosed in Japanese Patent Publication No. 2010-2581, a charging roller is sustained by a casing (or a limiting member). However, since a position of the charging roller is changed within the casing, complicated positional control may be required. Also, since the charging roller always needs to be in contact with the casing, a surface of the charging roller may be worn away and contaminated due to the casing.
Moreover, in the technique disclosed in Japanese Patent Publication No. 08-146709, pressure applied to ends of a conductive tube in an axial direction may differ from pressure applied to a center thereof in the axial direction. In particular, since relatively low pressure is applied to the center of the conductive tube, uniform charging cannot be performed along an axial direction of a charging roller. Also, when a weight is applied to a photosensitive body, an internal rubber ring may be deformed and thus an originally weak nip balance may be easily lost.